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Factory LSD lock check and rebuild question


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Hi guys,

 

I recently bought a factory 40% LSD 3.64 and I want to check its condition.

 

I saw the topic about the factory LSD rebuild, but the thing is that:

 

1. I want to find out what is the current lock ratio after the usage. Probably most of you will say that the discs are worn out after all the years, but before I break the diff for rebuild i am still curious to see what is the current lock ratio - after all if i am lucky the diff could be refurbished during the years. 
Is there something like bench test, like simulation of driving conditions how i can see the current lock rate. And i mean before breaking the diff into parts to measure the disc thickness. Currently i cant install it on the car either. Probably a stupid question, but...

 

2. How many inner and outer discs do these differentials with 40% lock up should have. And what is the size of each of them - i see sizes 1.9 / 2.0 / 2.1 mm? 


btw it was very interesting that when i turn one of the sides the other one runs on the opposite side, it acts like an open diff, then i understood it is specific for 40% lsd’s...

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785D6532-6C98-48D9-A7D4-74CA53A8A948.jpeg

19CECE0A-E22F-4F0B-8FFF-AB44945243F6.jpeg

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If I remember correctly you can check lsd lockup but you need special tools. If you take it to a differential shop they might be able to measure it for you.

 

That looks pretty clean inside the case. I would drain the fluid, inspect the gears for any chipping and/or cracking. As long as the gears check out ok and you don't find a lot of friction material in the old fluid. I would just install it and test it the old fashioned way, with a burnout.

 

If you plan on rebuilding it anyways just compare the old friction discs you remove from it to the new ones to get an idea of how much wear it had on it.

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This looks like an original 2002 LSD. These are slightly different to BMWs later LSD designs in that they don't rely on belleville washers (preload shims) to pre-load the clutches, but instead on flat shims and the exact stack-up height of the clutches & ramps.

The fact that you say you can turn one shaft  and the other turns the opposite direction already indicates that this pre-load is gone due to wear of the clutches. The diff may still offer some limited slippage in high torque situations, as in those situations the ramps are forced outwards to 'squeeze' the clutches even if theres  a few hundred micron of wear. I dont know how you simulate those levels of torque on the bench.

 

Remember the lockup percentage is defined by the ramp angle, not any attribute of the clutches. The more 'race spec' diffs have more preload which gives an earlier onset of slip limitation but also rapidly accelerates clutch wear.

 

Theres one clutch disc either side in the standard S40.

 

Before deciding on 1.9/2.0/2.1 discs you need to open the LSD to inspect and measure (details in the WSM). If the ramps are damaged, this can be 'fixed' with a ramp saver:

https://racingdiffs.com/products/bmw-ramp-saver-shim

the LSD shims are available from W&N.

With everything clean, measure the clutch disc thickness and the stack-up height in the LSD body...the thicker clutch helps compensate 100um of wear, the thinner clutch gives space for a ramp saver.

Its more tricky to get the preload right with these flat shim designs, you will probably have to try a few times...just have a selection of preload shims to juggle with.

Edited by dlacey
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I have an ‘02 LSD in my tii. It came from another tii. I bought it as an open differential — maybe $150 — because the locking was down to zero, and the seller assumed it was an open differential because the output flanges turned in opposite directions.

 

Korman opened it, saw it was an LSD, and wondered how I was so clever to find an ‘02 LSD! 😋🤫

 

I told the seller and sent him a few more bucks!

 

Regards,

 

Steve

 

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23 minutes ago, Conserv said:

I have an ‘02 LSD in my tii. It came from another tii. I bought it as an open differential — maybe $150 — because the locking was down to zero, and the seller assumed it was an open differential because the output flanges turned in opposite directions.

 

Korman opened it, saw it was an LSD, and wondered how I was so smart to find an ‘02 LSD! 😋🤫

 

I told the seller and sent him a few more bucks!

 

Regards,

 

Steve

 

Nice story. :) I guess this is my case too - worn discs and this is why the flanges are turning in opposite directions. I sent email to WN too, to find out for sure how many discs do I need and what size.

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13 hours ago, dlacey said:

 

f KP is 'KiloPonds' and this is same as 'Kilograms' (internet thinks so https://www.sizes.com/units/kilopond.htm), then 50cmkp=50cmkg=3.6 ft-lb   aka not much

https://www.onlineconversion.com/torque.htm

 

This is the BMW 1500/2000 manual, its a different diff but similar general design:

 

image.png

 

5-10 mkp = 36-72 lb-ft

A more reasonable figure... in the range we might expect....

 

 

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23 hours ago, BarneyT said:

Can someone tell me what I have?  What the number on top?  I’m putting this one in my 72 Ceylon Tii 

81E85C62-CD32-4649-9D2F-234EC9E58DE1.jpeg

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Barney, other than the antique “S” stencil on the right front flank, which signifies an e21-era limited-slip differential, your photos don’t show much of the differential.

 

Is the left front flank stamped with the number of ring and pinion teeth, and a month and date of manufacture? Are the side covers 4-bolt or 6-bolt?

 

We need more data to determine — I hope there is more data!

 

Below, the 1978 e21 LSD currently residing in my ‘76. The housing was cast April 26, 1978 (“26478” cast into top rear of housing). It’s a limited slip (stamped-in “S” between the number of pinion and ring gear teeth on the left front flank). It’s a 3.64 (“11” and “40” teeth stamped into the left front flank, divide 40 by 11 and get 3.64). The manufacturing date — in contrast to the housing’s casting date — is May 1978 (“5” over “8” stamped into the left front flank, just below the numbers of teeth). It has 4-bolt side covers, so it’s a pre-1979 unit.

 

Warning, not all differentials have all this information. The stenciled numbers on the top rear of your unit’s housing may replace cast-in numbers and may, or may not, be decipherable.

 

Best regards,

 

Steve

 

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836D050F-4B60-44E1-8F32-18BCCDB1AD67.jpeg

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4 hours ago, BarneyT said:

Can someone tell me what I have?  What the number on top?  I’m putting this one in my 72 Ceylon Tii 

81E85C62-CD32-4649-9D2F-234EC9E58DE1.jpeg

BEF1FADD-D865-49DB-98EC-9B8D9E1754AE.jpeg

The bolt hole threads at 5, 6 & 7 o'clock locations are in shabby shape.  Been drilled for 2 sizes holes.

Rotate the pinion and count the turns for 1 axle revolution to confirm the ratio and stamped numbers, gears may have been swapped.

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